This invention relates generally to headlights. More specifically, it relates to headlights which are permissible for use in potentially explosive atmospheres such as underground coal mines where explosive concentrations of methane may exist at times.
Headlights for underground coal mining machinery must be of very strong construction to contain an explosion in the event a methane-air mixture seeps in and is ignited by an electrical arc or heat within the housing. Temperature rise, and a dynamic gas effect called "pressure piling", produced by exploding a stoichiometric methane-air mixture in a sealed enclosure such as a headlight housing will cause a sudden pressure increase well in excess of 150 psi. It is therefore evident that enclosures for explosion-proof electrical equipment must be of much heavier construction than those commonly used for general consumer or non-mining industrial applications. A headlight housing for use in explosive atmospheres may be only six to eight inches in diameter but the walls must be one-fourth to one-half inch thick, with multiple reinforcing ribs, to prevent rupture and possible explosion of an entire mine section if the gas within the housing is ignited.
In addition to the high strength requirements, a mine headlight must be easy to take apart and re-assemble in constricted spaces where visibility is poor. The need for bulb replacement and internal maintenance from time to time make it impractical to permanently seal the housing, however desirable that might be from the standpoint of safety.
Engineers confronted with the problem of making a headlight strong for safety, yet capable of being taken apart for bulb replacement and maintenance, have properly been unwilling to trade off the safety of a rugged housing for features which might make it easy to change the bulb. As a result, headlights are basically safe for use in explosive atmospheres, but bulb replacement is often a substantial chore.